1. Field of the Invention
This invention relates to tubeless tires for large vehicles such as trucks and buses and more particularly to radial tubeless tires having an improved bead structure.
2. Prior Art
In radial tires for trucks and buses, steel cords are generally used both for a carcass ply layer and a belt layer. For tubeless tires, however, steel cords have the following disadvantages. Namely, pneumatic tires having inner tubes are liable to permit the inner air to escape from the inner tube and to lead outside through the aperture between the tire casing and the inner tube. This means that there is only slight chance of penetration of air and moisture included therein into the tire casing. On the contrary, the tire casing of a tubeless tire is open to the penetration of air and moisture contained therein. The air and moisture penetrating into the tire casing are retained in the spaces of the steel cords. As a result, the steel cords are rusted and in addition the adhesion between the steel cords and rubber compound is damaged.
Instead of steel cords, textile cords such as polyester cords having a strength comparable to steel cords are also used. The number of plies is increased so as to maintain the strength equivalent to that of steel cords. Increasing the number of plies results in an increase of the thickness of the inner-bead portion, i.e. that portion of the bead between a bead wire and a bead base mounted on a rim. Also, a bead seat of a rim used for tubeless tires for trucks and buses is inclined at 15.degree. and the height of the rim flange is very small. Therefore, when a bead wire is positioned outwardly in a radial direction due to an increase of thickness of the inner-bead portion, the radial outward position of the bead wire almost exceeds that of rim flange. As a result, separation of a carcass ply layer adjacent the inside of bead wire occurs.
In case the amount of rubber of the inner-bead portion between the bead wire and bead base is reduced, so as to position the bead wire radially closer to the rim, the bead rubber is more quickly worn by the rim during use of the tire. Consequently, the ply cords will be damaged and a blow-out may occur.
Furthermore, when the amount of rubber at the inner-bead portion is reduced, the compressibility required for mounting the tire on the rim is reduced. An airtight seal of a tubeless tire is achieved by mounting a bead portion of the tire on a bead seat of the rim under conditions such that the inner-bead portion located between the bead wire and bead seat is compressed. As a textile cord is only slightly compressible, when the inner-bead portion is occupied substantially by textile cords instead of rubber material, the bead will not have sufficient compressibility for mounting the tire on the rim. Thus, the tire can not easily be mounted on the rim or the mounting condition will be uneven around the circumference of the rim, thus resulting in the lack of an air tight seal, and also undesirable vibration may occur.